The adsorption of contaminants that are present in vanishingly low concentration—parts per million or below—presents a significant technical challenge in both environmental and industrial settings. To achieve meaningful adsorption capacity, an extremely high enthalpy of adsorption (ΔHads) is required. This energetic requirement typically lies well outside the range of physical adsorption processes, and will instead require the development of materials that interact chemically with the analyte of interest. Noteworthy progress has been achieved in the selective adsorption of 390-400 ppm carbon dioxide, as a first step towards its direct capture from air (Goeppert, et al., J. Am. Chem. Soc. 2011, 133:20164; McDonald, et al., J. Am. Chem. Soc. 2012, 134:7056; and Didas, et al., ChemSusChem 2012, 5:2058), and presents an interesting conceptual approach (Demessence, et al., J. Am. Chem. Soc. 2009, 131:8784; McDonald, et al., Chem. Sci. 2011, 2:2022; Lu, et al., Chem., Int. Ed. 2012, 51:7480; and Das, et al., Microporous Mesoporous Mater. 2013, 174:74). Appropriate adsorption enthalpies are achieved in these cases through multiple chemical interactions: initial interaction of a basic amine with the electrophilic carbon of CO2 yields a carbamic acid, which is further stabilized through either full proton transfer to yield an ammonium carbamate ion pair (Danon, et al., Chem. C 2011, 115:11540), or through hydrogen bonding interactions (FIG. 2A) (Planas, et al., J. Am. Chem. Soc. 2013, 135:7402). A similar approach can be envisioned, whereby multiple acidic sites located in close proximity result in the strong adsorption of Lewis basic pollutants (FIG. 2B).
Basic gases—such as ammonia—can lead to significant environmental and industrial concerns at similarly small concentrations. With regards to human health, ammonia itself has a recommended CAL-OSHA permissible exposure limit of only 25 ppm, which can be encountered in numerous industrial settings (http://www.dir.ca.gov/title8/5155table_ac1.html, CAL/OSHA. Acessed on Oct. 3, 2013). Highly toxic amines have even more stringent safety limits (e.g., diethanolamine, 0.46 ppm). In perhaps the most extreme example, volatile organic amines can disrupt photolithography at only tens of parts-per-billion concentration, resulting in characteristic ‘T-top’ channel features that render the resulting silicon wafer useless (FIG. 2C) (MacDonald, et al., Chem. Mater. 1993, 5:348; and Lin, et al., Aerosol Air Qual. Res. 2010, 10:245). As integrated circuits with narrower feature sizes are pursued, air purity requirements will become even more stringent (Kitajima, et al., IEEE Trans. Semicond. Manuf. 1997, 10:267). Outside of this specific application, ammonia adsorption can also serve as the first model for a generic acid-base interaction, taking the place of important but difficult to handle analytes such as the V-series of nerve agents (e.g., VX 1, FIG. 2D).
Currently, dense inorganic materials and unstructured polymers are commonly used technologies for the absorption of ammonia and amines. For transportation applications, simple salts such as MgCl2 are used, which absorb ammonia to produce coordination complexes of the type: Mg(NH3)xCl2, where x=1-6. While this material has a gravimetric capacity that will be difficult, if not impossible, to match, significant volume changes during loading and unloading, and the requirement for significant heat exchange during cycling, represent possible avenues for improvement.
Over the past fifteen years, metal-organic frameworks (MOFs) have demonstrated their utility in numerous applications, including gas storage, molecular separations, sensing, and size-selective catalysis (O'Keeffe, et al., Chem. Rev. 2012, 112:675; Getman, et al., Chem. Rev. 2012, 112:703; Sumida, et al., Chem. Rev. 2012, 112:724; Suh, et al., Chem. Rev. 2012, 112:782; Li, et al., Chem. Rev. 2012, 112:869; Wang, et al., J. Am. Chem. Soc. 2011, 133:13445; and Yanai, et al., Nature Mater. 2011, 10:787). Some preliminary investigations into the use of these materials for ammonia adsorption have also been conducted. The overwhelming majority of these examples use Lewis acidic framework sites to increase the strength of adsorption for NH3. In materials such as MOF-74, exposed metal cations provide the desired adsorption sites (Glover, et al., Chem. Eng. Sci. 2011, 66:163). In related materials known as covalent organic frameworks (COFs), it has been demonstrated that three-coordinate boron centers can behave in a similar Lewis-acidic fashion, with the framework generated from hexahydroxytriphenylene and biphenyldiboronic acid (COF-10) displaying high uptake at moderate pressure (Doonan, et al., Nature Chem. 2010, 2:235).
In the context of MOFs, the use of Brønsted acidic centers for the adsorption of ammonia has been explored to a much lesser degree. Trikalitis reported a MOF-205 (Furukawa, et al., Science 2010, 329:424) analog containing free phenolic —OH groups, which demonstrated excellent low and moderate pressure ammonia capacity (Spanopoulos, et al., Colloid Interface Sci. 2010, 348:615). However, the basic zinc acetate-type structure was not stable to ammonia exposure, with framework collapse suggested by powder X-ray diffraction and gas adsorption experiments, in line with previous observation made on analogous zinc-based materials (MOF-5 and MOF-177) (Saha, et al., Colloid Interface Sci. 2010, 348:615; and Petit, et al., J. Adv. Funct. Mater. 2010, 20:111). In an effort to generate ammonia adsorbents that would be stable, and therefore potentially reusable, Yaghi investigated a zirconium-based UiO-66 analog (Cavka, et al., J. Am. Chem. Soc. 2008, 130:13850) featuring anilinium cations as the Brønsted acid source (Morris, et al., Inorg. Chem. 2011, 50:6853). Although only one-third of the available aniline sites in the material had been protonated, a meaningful increase in NH3 adsorption was noted, and the framework survived exposure up to 1 bar of pressure.
Recently, porous aromatic polymers featuring a diamondoid-type structure have been introduced as ‘element-organic frameworks’ (e.g., EOF-1) (Rose, et al., Chem. Commun. 2008, 2462), ‘porous aromatic frameworks’ (e.g., PAF-1) (Ben, et al., Chem., Int. Ed. 2009, 48:9457), and ‘poly(aryleneethynylene) networks’ (e.g., PAE-E1) (Stockel, et al., Chem. Commun. 2009, 212), with initial investigations on ammonia capture using isolated metal catechol (Weston, et al., Chem. Commun. 2013, 49:2995) and polyimide (Peterson, et al., Porous Mater. 2012, 19:261) functional groups reported.
Porous aromatic frameworks have attractive features, and a polymer based on this type of framework that was able to adsorb ammonia and/or amines would represent a significant advance in the art concerned with removing these types of toxic substances from the ambient atmosphere. Through provision of a novel PAF in which Brønsted acid moieties are incorporated into this framework, the present invention satisfies this and other needs.